The Chinese. All things considered, no one was going to support Japan's monopolization of China, save Hitler. The Allies could simply pour more Lend Lease into supporting the United Front and Japan would continue to be bogged down in an unwinnable war in the countryside. It's not impossible for Japan to win, but that required a West that didn't have vested interest in Chinese markets, an unwillingness to have their access to them hampered by Japanese tolls and resource hoarding, or revulsion for Japanese atrocities against the civilian population.